Typical communication systems include infinite impulse response (IIR) filters for suppressing energy at selected frequencies. Such a filter comprises one or more individual filter sections with each section having a pair of multipliers connected in parallel with an adder. The specific characteristics of any given filter section are determined by the values of the multipliers. For example, the multiplier values may be chosen so as to perform a band stop function for high band frequencies or a band stop function for low band frequencies.
The selection of the specific values of the multipliers in a filter section is well documented in the prior art for a large number of fixed filter functions. The selection of these multiplier values must be carefully made because the multipliers are in a feedback path. If the selection of the multiplier values is done without some care, unstable operation of the filter can result. The potential instabilities associated with multiplication in a feedback system is also well known in the prior art. See for example, Feedback Control System Analysis and Synthesis, D'Azzo and Houpis, McGraw-Hill, N.Y., 1960.
There are several applications which require the suppression of energy at time varying or ensemble varying frequencies. Examples of such applications include the detection of call progress signals on a telephone line; carrier detection in a modem; discrimination in a modem; automatic equalization of predistorted signals; and, the detection of dual tone multi-frequency (DTMF) tones in a telephone system. For these time varying or ensemble varying frequencies, fixed filters may not be suitable.
U.S. Pat. No. 4,223,272 discloses a four terminal network selectively convertible into a low pass, high pass, band pass, all pass, or band stop filter. The four terminal network has a transfer function substantially satisfying a quadratic equation, wherein the coefficients of the quadratic equation may be changed to alter the function of the network. The coefficients of the quadratic equation are altered by changing the values of resistances in a network. These resistances are switchable but only to the extent of converting the four terminal network from one form of filter to another. The resistances are not dynamically switchable to alter the filter characteristics on a dynamic basis for either a time varying or ensemble varying frequency.
U.S. Pat. No. 4,182,997 describes a filter for a telecommunication system designed to suppress a first signal frequency and to transmit an adjoining second signal frequency without significant attenuation. The frequencies at which the pass and stop functions are performed depend upon the values of a first and a second capacitor and a first and a second inductor. Similarly, U.S. Pat. No. 3,531,652 discloses an active filter circuit that is adapted to filter functions to pass a source frequency with very little attenuation while attenuating other frequencies. Additionally, U.S. Pat. No. 3,628,057 discloses an active narrow notch filter to filter out noise signals. However, none of these prior art references discloses a filter which is dynamically variable so as to suppress energy at either time varying or ensemble varying frequencies.